1. Field of the Invention
The present invention relates to a fluxing curative for an underfill solution that is used to assemble an electronic package. More particularly, the fluxing curative, during a solder operation, fluxes the solder and/or solder pads and cures the epoxy resin of the underfill solution.
2. Description of Related Technology
Electrical components such as resisters, capacitors, inductors, transistors, integrated circuits, and chip carriers are typically mounted on circuit boards according to one of two configurations. In the first configuration, the components are mounted on one side of the board and leads from the components extend through holes in the board and are soldered on the opposite side of the board. In the second configuration, the components are soldered to the same side of the board upon which they are mounted. These latter devices are said to be “surface-mounted.”
Surface mounting of electronic components is a desirable technique in that it may be used to fabricate very small circuit structures and in that it lends itself well to process automation. A type of surface-mounted device referred to as an area array package comprises an integrated circuit having numerous connecting leads attached to pads mounted on the underside of the device. Examples of area array packages include a flip chip, a chip scale package (CSP), and a ball grid array (BGA). In connection with the use of an area array package, either the circuit board or the device is provided with small bumps or balls of solder (hereinafter “bumps” or “solder bumps”) positioned in locations which correspond to the pads on the underside of each device and on the surface of the circuit board. The device is mounted by (a) placing it in contact with t he board such that the solder bumps become sandwiched between the pads on the board and the corresponding pads on the device; (b) heating the assembly to a point at which the solder is caused to reflow (i.e., melt); and (c) cooling the assembly. Upon cooling, the solder hardens and affixes the area array device to the board's surface. Tolerances in area array technology tend to be critical because the spacing between individual devices as well as the spacing between the chip and the board is typically very small. For example, spacing of a flip chip from the surface of a board to the bottom of a die is typically between about 15 and about 75 μm and is expected to approach about 10 μm in the near future.
One problem associated with area array technology is that the chips, the solder, and the material forming the circuit board often have significantly different coefficients of thermal expansion. As a result of the differing expansions, the heating of the assembly during use can cause severe stresses. The stresses imposed on the solder interconnects can lead to failures that degrade device performance or incapacitate the device entirely.
In order to minimize thermomechanical fatigue resulting from different thermal expansions, thermoset epoxies have typically been used. Specifically, these epoxies are used as an underfill material that surrounds the periphery of the area array device and occupies the space beneath the chip between the underside of the chip and the board which is not occupied by solder. Such epoxy systems provide a level of protection by forming a physical barrier that resists or reduces the stress on solder interconnects due to different expansions among the components of the device and/or to drop shocks.
Improved underfill materials have been developed in which the epoxy thermoset material is provided with a silica powder filler. By varying the amount of filler material, it is possible to cause the coefficient of thermal expansion of the filled epoxy thermoset to more closely match that of the integrated circuit and printed circuit board substrates. In so doing, relative movement between the underside of the flip chip and the solder connections, resulting from their differing coefficients of thermal expansion, is minimized. Such filled epoxy thermosets therefore reduce the likelihood of device failure resulting from thermomechanical fatigue during operation of the device.
While underfill has solved the thermal mismatch problem for area array devices on printed circuit boards, it has created significant difficulties in the manufacturing process. For example, the underfill must be applied off-line using special equipment. Typically, the underfill is applied to up to three edges of the assembled flip chip and allowed to flow all the way under the chip via capillary action. Once the material has flowed to opposite edges and all air has been displaced from under the chip, additional underfill is dispensed to the outer edges so as to form a fillet making all four edges symmetrical. This improves the reliability and the appearance of the assembly. Next, the assembly is baked in an oven to harden the underfill. This process, which may take up to several hours, is necessary to harden and fully cure the underfill. Thus, although the underfill couples the area array device to the substrate replacing shear stresses with bending stresses, and provides a commercially viable solution, a simpler manufacturing method is desirable.
Recently, attempts have been made to improve and streamline the underfill process. One method that has shown some commercial potential involves dispensing underfill before assembling the area array device to the substrate and making solder connections. This method requires that the underfill allow solder joint formation to occur. Soldering of area array devices to printed circuit boards is generally accomplished by applying flux to the solder bumps on the device or to the pads on the printed circuit board. Thus, the flux must be applied to the bumps before the underfill or the underfill must contain flux or have inherent properties that facilitate solder joint formation. Flux activity is needed to remove the oxidation on the pads for the solder to wet the pad metalization forming acceptable interconnects.
Certain underfills commonly called “dispense first underfills,” “pre-applied,” “package applied,” or “no flow” underfills have been designed with self-contained flux chemistry. Unfortunately, the properties required for a good flux and those required for a good underfill have tended to be incompatible. As such, a compromise of properties has been the result. For example, the best fluxing underfill materials typically require more than an hour to harden or are too reactive and gel prematurely, which results in electrical disconnects. More specifically, premature gellation may result in misalignment between the device and the substrate and/or it may prevent collapse of the solder bumps, either of which prevents the formation of an adequate solder connection and may render the electronic assembly defective or inoperable.
In an attempt to alleviate some of the foregoing problems, fluxing underfill compositions containing anhydride compounds as a curing agent have been developed. Advantageously, underfills utilizing an anhydride curing agent tend to have a cure profile that is compatible with conventional solder reflow operations. However, the use of anhydride compounds has resulted in other drawbacks to no-flow solder systems. For example, anhydride compounds tend to produce residues that are corrosive to the metal pads and/or the solder alloy, which may result in the electronic assembly becoming inoperable. Additionally, anhydride compounds tend to form voids that degrade the strength and adhesion of the underfill especially at the high reflow temperatures needed for lead free solder applications. Still further, anhydride compounds tend to take part in the bonding or crosslinking of epoxy molecules with ester linkages, which are susceptible to degradation when exposed to moisture. This degradation of the epoxy decreases the reliability performance of the underfill and may result in failure of the electronic assembly.